USPS first class mail return to sender?

Hi, so i recently sold something to someone on eBay, Now they told me that wanted to return it and i just found out that they lost my return label and instead taped over the original tracking number to cover it and just wrote Return to Sender on the package. Surely this wouldn't work? The guy says he put it into the mailbox a few days ago. I'm not sure what to do at this point.